1. Field of the Invention
The present invention relates to an image forming apparatus, a method of correcting image magnification, and a computer-readable recording medium.
2. Description of the Related Art
When an image forming apparatus performs double-side printing on a printing medium, the printing medium is expanded or contracted due to influence of heat given at the time of printing a first surface (front surface). Therefore, the size of the printing medium may be slightly changed at the time of printing a second surface (back surface) as compared with the size at the time of printing the first surface. Such a change in size of the printing medium is a factor of misregistration of the front and the back of printed matter generated through double-side printing. Due to this, a method is known for detecting each size of the printing medium before the first surface is printed and before the second surface is printed to calculate a ratio of expansion or contraction, and correcting magnification of an image to be printed on the second surface corresponding to the ratio of expansion or contraction (for example, refer to Japanese Laid-open Patent Publication No. 2007-72094, Japanese Laid-open Patent Publication No. 2007-79262, and Japanese Laid-open Patent Publication No. 2009-42461).
The ratio of expansion or contraction of the printing medium mainly depends on a type of the printing medium and a temperature of the image forming apparatus that varies corresponding to elapsed time from the start of printing. Also known is a method of setting a representative value of the ratio of expansion or contraction that is estimated corresponding to the type of the printing medium as an initial value in a storage unit in advance, and correcting the magnification using the latest value of the ratio of expansion or contraction held by the storage unit while updating the storage unit with the value of the ratio of expansion or contraction that is calculated as needed in printing. In this example, when a difference between a newly calculated value of the ratio of expansion or contraction and a value held by the storage unit exceeds a predetermined range, the storage unit is not updated and the newly calculated value of the ratio of expansion or contraction is discarded. This is because the size of the printing medium is considered not to be correctly detected according to the fact that the newly calculated value of the ratio of expansion or contraction varies largely from the previous values, so that reliability of the calculated value of the ratio of expansion or contraction is low. When the magnification is corrected using the largely varied value of the ratio of expansion or contraction, variation in size of the image is noticeable among printing media that are successively printed, which should be prevented.
In the above method of correcting the magnification corresponding to the latest ratio of expansion or contraction held by the storage unit, the magnification can be appropriately corrected following a change in temperature of the image forming apparatus so long as a representative value of the ratio of expansion or contraction that is estimated depending on a type of the printing medium is set as an initial value in the storage unit first. The representative value of the ratio of expansion or contraction set as the initial value needs to be obtained by performing double-side printing in advance under the same condition as in actual printing and measuring the size of the printing medium. The operation of obtaining the representative value of the ratio of expansion or contraction in advance is complicated. In particular, a large number of types of printing media are provided in recent years, so that it is very complicated to obtain respective representative values of the ratio of expansion or contraction for all types of printing media in advance.